Various springs have conventionally been adopted in precision machines such as a timepiece and a music box. In a timepiece, for example, there are known a spring fixing a crystal oscillator of a crystal oscillating timepiece in a wielded state, a mainspring composing a power source for a driving mechanism of a timepiece, a click spring provided for preventing back-winding upon winding a mainspring, and a hairspring wielding a timed annular balance in a mechanical timepiece.
Conventional materials applicable for these springs include spring materials and mainspring materials such as carbon steel, stainless steel, a cobalt alloy, and a copper alloy. These materials have however the following problems.
1. The spring fixing a crystal oscillator in a wielded state poses a problem in that the wielding force of the spring causes a shift in the pace of the crystal oscillator. More specifically, dispersion of the spring wielding force causes a gain or a loss of the period of a 32 kHz signal issued by the crystal, and this leads to a problem of a shift of accuracy of a timepiece using this signal as a reference signal. The smallest possible dispersion of wielding force is therefore required for a spring fixing the crystal oscillator.
2. In a hairspring wielding a timed annular balance forming a governor for a mechanical timepiece, a temperature change results in a change in Young's modulus which in turn causes dispersion of the wielding force, and hence a change in the oscillating period of the timed annular balance. This change in the oscillating period of the timed annular balance exerts an important effect on the accuracy of a mechanical timepiece. It is therefore desirable to adopt a hairspring material, of which Young's modulus does not change under the effect of a change in temperature.
3. Further, in the case of a mainspring serving as a power source for a driving mechanism of a timepiece or the like, a mainspring satisfying contradictory requirements of a long-time operation of the driving mechanism and downsizing of the driving mechanism is demanded. More specifically, for example, a driving mechanism of a timepiece comprises a mainspring serving as a power source, a barrel drum housing the mainspring, and a train wheel transmitting a mechanical energy of the mainspring by engaging with the barrel drum. Hands of the timepiece are rotated, via a transmitting unit such as the train wheel, by the use of the rotation force produced by the release of the tightly wound mainspring.
The number of turns of the mainspring serving as a power source of such a driving mechanism and the output torque are in a proportional relationship. When the output torque of the mainspring is T, the number of winding runs (number of turns) of the mainspring is N, Young's modulus is E, the total length of the mainspring is L, and the mainspring is assumed to have a rectangular cross-section having a thickness t and a width b, it is known that T can be expressed by:T=(Et3bπ/6L)×N  (1)
On the other hand, the total length L, the thickness t and the width b of the mainspring are dependent on the size of the barrel drum housing the mainspring. If the barrel drum has an inside radius R and a barrel arbor radius r, the total length L of the mainspring is determinable from the following formula:L=π(R2−r2)/2t  (2)It is thus suggested that the total length L and the thickness t of the mainspring are in a inversely proportional relationship.
The mechanical energy accumulated in the mainspring is obtained by integrating the output torque of Equation (1) by the number of turns N, and Equation (1) is considered to be a function of the total length L and the thickness t of the mainspring. The spring energy has therefore conventionally been adjusted by controlling L and t.
This means that the maximum number of turns Nmax of the mainspring can be increased by reducing the mainspring thickness t and increasing the mainspring total length L.
On the contrary, the value of output torque T can be increased by reducing the total length L of the mainspring, and increasing the mainspring thickness t.
As is evident from Equation (2), however, in this manner of determination, the mainspring thickness t and the total length L are limited by the volume of the housing space within the barrel drum. When adopting a mainspring operable for a long period of time, therefore, it is necessary to use a larger-sized barrel drum and a larger housing space, thus leading to a problem of impossibility to downsize the driving mechanism including the mainspring.
It was once conceived to achieve a mainspring capable of outputting a high torque with a thinner thickness t by adopting a mainspring material having a high Young's modulus. This contrivance was however limited in terms of mainspring durability since it was difficult to maintain toughness of the mainspring.
The present invention has an object to provide a spring which permits achievement of a high accuracy and stable operation of a precision machine such as a timepiece, and to provide a spring enabling, when used as power source, to operate for a long period of time, and a driving mechanism having this spring as a power source.